Sign-ups don't open until the park opens, though we saw a line with a couple of families in it as early as 7:10 during our MM. The sign-up line is outside the Star Wars Launch Bay. It started outside the entrance, though they eventually moved us up the ramp to the second floor ramp around 7:50. If they do that, the line will look non-existent, so don't be fooled: it's just further up the ramp.
Getting in line about 10 minutes before the end will work, though if there's someone in your party who wouldn't mind getting in line a little earlier and holding a spot that might help so long as the kids are in line with you by the time the park opens. In our case, DH and the girls wanted to go on Star Tours, but I can't because I get motion sick too easily. I offered to wait in line for Jedi training (around 7:40 or 7:45) while they did Star Tours, which only had a 10-minute wait. Then DH walked them up to the ramp where I was in line, and they got there just as they opened the sign-ups at 8 am. We probably had 10 families in front of us, so it still took another 5-8 minutes before they got to the kids. I think there are about 15 kids per class.
